Cermet potentiometers are characterized by a thick film resistance element deposited over a substrate which is typically alumina and a contact device or wiper which takes the form of a strip-like array of wires each acting as an individual contact finger. The cermet consists of a mixture of glass frit and precious metal particles suspended in a organic type emulsion solution which is silk-screened onto the substrate and then fired. The organic carrier burns off leaving a glass with the metal particles suspended. The ratio of metal particles to glass determines the resistivity of the cermet after firing.
The multifinger contact designs used in cermet potentiometers exhibit lower contact noise than single contact designs because the individual fingertips track the surface of the cermet independently. This eliminates much interruption of contact and the "make and break" noises associated with such interruptions. An example of such an array may be seen in U.S. Pat. No. 3,704,436 to Froebe et al, issued Nov. 28, 1972. The multifinger design provides greater uniformity of contact area and pressure as the wiper passes across the resistance element, resulting in less contact resistance and contact noise. For those skilled in the art such noise is known as contact-resistance-variation or CRV.
As it has been for many years, the uniform practice in the art today is to provide a small semicircular bend at the free end of each contact finger so that the fingers track the resistance element along its side or circumferential surface, as for example illustrated in the Froebe et al patent. While this has been a longstanding practice in cermet potentiometer design the rather severe abrasiveness of the resistance elements makes the practice less than ideal because a measureable portion of the contact finger is actually ground off during each pass of the wiper across the element. Unfortunately, what starts out as a relatively small elliptical area of contact between the abrasive cermet surface and the curved side of each wire finger quickly enlarges as the wiper scrapes back and forth over the abrasive surface of the resistance element. Obviously, the abrasion occuring during each turn of the potentiometer causes all the elliplical contact zones to systematically increase while their contact pressures must decrease due to the constancy of the applied spring force. This translates directly into a systematic drift in noise characteristics and possibly in calibration as well, both of which are undesireable in most applica- tions.
Moreover, the contact fingers can also fail catastrophically when the wires are worn substantially or entirely through from one side to the other. Of course, catastrophic failure of this nature when the potentiometer is in an electrical system is always bothersome and even potentially dangerous as one well might imagine.